1. Technical Field of the Invention
The embodiments of the invention relate to wireless communications and, more particularly, to frequency compensation of clocks and/or oscillators for a wireless mobile device.
2. Description of Related Art
Various wireless communication systems are known today to provide communication links between devices, whether directly or through a network. Such communication systems range from national and/or international cellular telephone systems, the Internet, point-to-point in-home systems, as well as other systems. Communication systems typically operate in accordance with one or more communication standards or protocols. For instance, wireless communication systems may operate using protocols, such as IEEE 802.11, Bluetooth™, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), as well as others.
Presently, in the mobile (e.g. cellular) telephone area, 3G (3rd Generation) mobile phones based on 3GPP (3rd Generation Partnership Project) technology utilize Evolved High Speed Packet Access (HSPA+) to obtain high data rates for downloads. HSPA+ allows data rates approaching 21 Mbps and, in some categories, may exceed 21 Mbps. The trend for mobile devices is to move toward Long Term Evolution (LTE) technology and 4G (4th Generation) technology to obtain much higher data rates.
For each wireless mobile communication device, such as a mobile phone, to participate in wireless communications, it generally includes a built-in radio transceiver (e.g., receiver and transmitter) or is coupled to an associated radio transceiver. Typically, the transceiver (or radio) includes a baseband processing stage and a radio frequency (RF) stage. The baseband processing provides the conversion from data to baseband signals for transmitting and baseband signals to data for receiving, in accordance with a particular wireless communication protocol. The baseband processing stage is coupled to a RF stage (transmitter section and receiver section) that provides the conversion between the baseband signals and RF signals. The RF stage may be a direct conversion transceiver that converts directly between baseband and RF or may include one or more intermediate frequency stage(s). For handheld devices, where most or all of the components are resident in the device, the handheld device typically also includes an application processor or processors to execute various applications for the device.
The radio portion of the handheld device, such as a mobile phone, uses a crystal oscillator to generate accurate clock signals at a reference frequency. In one instance, the crystal oscillator generates clock signals to further produce local oscillator output for use in signal conversion in the receiver and/or the transmitter. However, a drift in frequency causes a number of problems in a mobile (e.g. cellular) phone, such as carrier frequency synchronization and time synchronization. For example, a frequency drift of the crystal oscillator may cause a variation in the sampling time, so that the data is not sampled at the correct moment. Incorrect sampling can increase the bit error rate (BER) and reduce the throughput of the system. This impairment is more pronounced for higher data rate applications, such as HSPA+. To avoid the drift in the output frequency, mobile phones use some mechanisms to adjust the reference frequency generated by the crystal oscillator. In the past, the capability to maintain the target oscillation frequency was provided by a voltage-controlled, temperature-compensated, crystal oscillator (VC-TCXO). Low cost and smaller handsets now employ transceivers with fully integrated digitally-controlled crystal oscillators (DCXOs) at the clock interface. DCXOs use a low cost crystal and an array of capacitors to adjust the reference frequency.
DCXOs typically contain two arrays of capacitors. The first array of capacitors is used to provide coarse adjustment to compensate for static error due to process variations. The second array of capacitors is used to provide very fine tuning capability to compensate for any dynamic error, such as temperature drift, pushing or pulling impacts. For mobile phones utilizing low data rates, some amount of frequency drift is acceptable. However, for higher data rate phones, such as phones with HSPA+ capability or higher, drift of the reference frequency of even in a few Hertz may have substantial consequences. Achieving a few Hz of resolution within the full range of the capacitor arrays is difficult to obtain and, in some instances, could be limited by the accuracy of the fabrication process. Accordingly, these limitations and process variations may degrade the resolution of the DCXO and cause a significant frequency gap in a tuning curve of a DCXO. Any frequency gap could result in signal loss for a mobile phone, if the phone is operating or attempting to operate at a frequency that crosses the gap.
Accordingly, there is a need for a reference component, such as a DCXO, to be compensated for dynamic and static errors, as well as continuously adjusting the frequency to prevent the occurrence of frequency gaps, while maintaining overall reference frequency error resolution to be in the range of a few Hertz.